You have a choice of three different DACs for the XM6. All DACs are built by Wolfson Microelectronics.

DAC

WM8740

WM8742

WM8741

Price

+$0

+$20

+$42

Resolution

24 bit

24 bit

24 bit

Sample Rate

192 kHz

192 kHz

192 kHz

Published SNR

120dB

126dB

128dB

Published THD

-104dB

-100dB

-100dB

All three DACs have excellent audio performance. If you are relatively new to the headphone amplifier environment, or are looking to save a few dollars, we would recommend the WM8740 - it has very strong specs, and indeed edges out the others very slightly in THD performance.

The WM8741 and WM8742 are slightly newer chips from Wolfson, and similarly have excellent specs. If you are a headphone veteran, or experimenter, than by all means choose one of these upgrades.

For the truly hard-core, we've linked to Wolfson's product pages for each of the DACs. From here, you can download the Engineering datasheets for each: